The present invention relates to allocation of data transmission resources particularly in mobile communications systems.
In telecommunications systems, the parties to the transmission communicate with one another utilizing the transmission resources allocated to the system. The transmission resources are divided into channels. It is essential for the success of the data transmission that the channel used on the connection is free from interference and noise to such an extent that the receiver can sufficiently faultlessly interpret the information sent by the transmitting party from the signal received from the channel. Interference is constituted by interference signals caused by other connections to the channel, and it has particular importance in wireless systems, such as mobile communications systems.
In mobile communications systems, the mobile stations and base stations can establish connections via a radio interface. A specific frequency band has been allocated as a transmission resource. This frequency band is further divided permanently between various networks. In the context of the present application, the term network denotes a telecommunications network operated by one network operator according to one system. Each network divides the frequency band allocated for its use further into channels. In order that sufficient capacity may be obtained for the mobile communications system on this limited frequency band, the available channels must be reused several times. The coverage area of the system is therefore divided into cells constituted by the radio coverage areas of individual base stations, wherefore such systems are often also termed cellular radio systems.
The air interface between the base stations and mobile stations can be divided into channels in several ways. Known ways include time division multiplexing TDM, frequency division multiplexing FDM, and code division multiplexing CDM. In TDM systems, the available bandwidth is divided into successive time slots. A given number of successive time slots forms a periodically recurring time frame. The channel is determined by the time slot used in the time frame. In FDM systems, the channel is determined by the frequency used, and in CDM systems by the frequency hopping pattern or spreading code used. Also combinations of the above division methods may be used.
To maximize the capacity, it is attempted to reuse the channels in cells as close as possible, yet so that the quality of the connections using the channels remains sufficiently good. Connection quality is influenced by the sensitivity of the transferred information to transmission errors arising on the radio channel and by the radio channel quality. The transmission error tolerance of the signal is dependent on the properties of the transferred information, and it can be improved by processing the information prior to its transmission onto the channel by channel coding and interleaving and by using re-transmission of faulty transmission frames. The radio channel quality is represented by the carrier-to-interference ratio CIR, which is the ratio of the strength of the signal sent by the transmitter and the interference caused by other connections on the channel as experienced by the receiver on the connection.
The magnitude of interference caused by connections to each other is dependent on the channels used by the connections, the geographic location of the connections and the transmit power used. These factors can be influenced by planned channel allocation to different cells which takes interference into account, dynamic control of the transmit power, and averaging of the interference experienced by the different connections.
To maximize the utilization of the available transmission capacity, various channel allocation methods have been developed. The aim in channel allocation is to allocate for desired connections channels all of which can be utilized simultaneously whilst the signal quality remains acceptable. To maximize capacity, the channels should be reused as close as possible.
Known channel allocation methods include fixed channel allocation FCA, dynamic channel allocation DCA and hybrid channel allocation HCA, which is a combination of FCA and DCA. The idea in fixed channel allocation is to divide the channels available to the system between the cells already in frequency planning performed prior to the commissioning of the system. In dynamic channel allocation, all channels are in a common pool of channels, wherefrom the best channel is selected for use for the connection to be established on the basis of a predetermined norm. In hybrid channel allocation, some of the channels available to the system are permanently divided for the use of different cells as in FCA, and the remainder are placed in a channel pool wherefrom they can be taken dynamically as required for the use of any cell. The different methods have been thoroughly described in I. Katzela and M. Naghshineh, Channel Assignment Schemes for Cellular Mobile Telecommunications Systems: A Comprehensive Survey, IEEE Personal Communications, pp. 10-31, June 1996.
Known methods for equalizing interference between different connections include frequency hopping in FDM systems and time slot hopping in TDM systems. In CDM systems, interference between connections is equalized by using sufficiently dissimilar spreading codes. On the other hand, in the method all connections utilize the same frequency, which considerably increases the average of mutual interference.
In frequency hopping, the frequency of the connection is changed at frequent intervals. The methods can be divided into rapid and slow frequency hopping. In rapid frequency hopping, the frequency of the connection is changed more frequently than the carrier frequency used. In slow frequency hopping, on the other hand, the frequency of the connection is changed less frequently than the frequency of the carrier frequency used.
In the known GSM system, for instance, frequency hopping is implemented in such a way that an individual burst is always sent at one frequency, and the burst to be sent in the next time slot at another frequency. In such a case, an individual burst may experience a high interference level. However, on account of channel coding and interleaving it suffices for good connection quality that a sufficient portion of the bursts can be transferred without appreciable interference. Frequency hopping allows this condition to be fulfilled connection-specifically, even though some of the bursts were to suffer considerable interference.
Time slot hopping is based on a similar principle as frequency hopping. In time slot hopping, the time slot utilized on the connection is changed instead of frequency. Also the hopping patterns in time slot hopping should be independent of one another in cells located close to one another to achieve the best result.
The capacity of a telecommunications network is ultimately limited by the frequency band permanently allocated for the use of the network. In telecommunications, the capacity requirement is of a statistical nature. Calls are initiated and terminated independently of one another, as a result of which the traffic level varies. The quantity of traffic and the number of channels needed to meet the traffic level can be given a probability distribution. FIG. 1 shows an example of the probability distribution of the channel requirement. The figure considers the probability distribution of the channel requirement in a situation in which the time-dependent channel requirement is 24 channels on an average, and the standard deviation for the channel requirement is about five channels. The operator has 30 channels at his disposal. If the traffic poses a requirement of more than 20 channels, it is not possible to serve all users but blocking occurs. In the case of the figure, all 30 channels are in use 7.7 percent of the time, and thus a user attempting a connection in the operator""s network will experience blocking with a probability of 7.7 percent. If another operator is operative in the same area, having a similar number of channels at his disposal and experiencing a similar channel requirement, one of the operators will presumably have vacant capacity while the users of the other operator experience blocking.
On account of the statistical nature of traffic, the capacity of one network may be fully occupied in a given area, thus causing blocking of new calls that should be established, even though another network simultaneously has a large amount of unused capacity in that area. This situation is shown in FIG. 2, which illustrates the distribution and use of transmission resources in an area. In the figure, frequencies F1-F9 constituting the transmission resources are divided between three networks in such a way that network 1 has been assigned frequencies F1, F2 and F3 for its use, network 2 frequencies F4, F5 and F6, and network 3 frequencies F7, F8 and F9.
The channels used by the connections are shown as hatched in FIG. 2. The unhatched area depicts the idle channels. Of the resources allocated to network 1, the network utilizes frequency F1 in full and eight of the ten channels established at frequency F2. Frequency F3 is completely free. At the point of consideration, network 2 utilizes all frequencies F4, F5 and F6 allocated to it in full. Network 3 utilizes frequency F9 in full, {fraction (3/10)} of frequency F8, and frequency F7 allocated to it is free. Hence, in the situation shown in the figure the users of network 2 experience blocking, although all resources available in the area are not in use.
As the number of mobile subscribers increases and applications requiring wide bandwidth, such as multimedia applications, become more common, the prior art channel allocation methods are no longer capable of utilizing the available frequency spectrum efficiently enough. Special problems are presented by situations in which the limited frequency band is jointly used by several different systems, such as a mobile communications system and a cordless office system. It is an object of the present invention to alleviate these problems by rendering the allocation of transmission resources more effective. This object is achieved with the method disclosed in the independent claim.
The idea of the invention is the allocation of transmission resources in several separate steps. In the first step, the available transmission resources are dynamically divided between the different networks. In the second step, the networks divide the resources that have been allocated for their use among their users by their own channel allocation methods.
In one embodiment, a given minimum capacity wherewith the network achieves a predetermined minimum quality for its service is permanently allocated to some or all of the networks. As the capacity requirement increases, the necessary amount of additional capacity is allocated to the operator in excess of this minimum capacity. In such a case, the additional capacity is allocated either from resources separately reserved for this purpose that are common to the networks, or by borrowing it from capacity that is allocated to another network but falls outside the minimum capacity of said network.
In accordance with one embodiment, an upper limit is set for the transmission capacity allocated to a network, in excess of which no capacity can be allocated to the network.
The dynamic distribution of capacity between networks can be realized either in a centralized or in a distributed manner. If the division is performed in a distributed manner for example in such a way that each network independently allocates a band to itself, the algorithms used in the different networks must be compatible.
The amount of capacity to be allocated for the use of a network can be influenced for instance by the traffic load and the forecast on its behaviour in the immediate future, contracts between the operators, transmit power levels used, and results of measurements on radio path signals. On the basis of such measurement results, it can for example be concluded how great an increase additional capacity allocated to the network will make on the information transfer rate, in other words, what the frequency performance of the network is.